Can you try update to the latest driver/firmware - we have MLNX_OFED / EN 3.4.2 posted on the web.
I think you know it, but just in case, see HowTo Install MLNX_OFED Driver , but do it for Ubuntu (the post is for CentOS... the link)
Which cables are you using?
Try on and off the autoneg
I'm note sure what is the issue? do you have support with Mellanox?
ethtool -s enp2s0f0 speed 25000 autoneg off
ethtool -s enp2s0f0 autoneg on
I'm sure you did,just checking, did you try reset?
Ophir.